The present invention relates to a nozzle for spinning optically anisotropic pitch to prepare carbon fibers having high strength and high modulus of elasticity, and more in particular to a spinning nozzle for preparing pitch-based carbon fibers excellent in homogeneity having no defects such as wedge-like cracks parallel to the fiber axis.
Carbon fibers of a high performance grade prepared from optically anisotropic pitch possess such characteristics that the fibers can be prepared less expensively than PAN-based ones and high elasticity can be easily realized by means of graphitization. On the other hand, the pitch-based carbon fibers possess such drawbacks as low strength and low elongation so that the application thereof is rather limited.
Various researches and developments have been conducted to improve the above dynamical properties of the pitch-based carbon fibers. One of the researches and the developments is a method of treating precursor pitch which includes, for example, a method consisting of discharging light components which prevent formation of mesophase to depress excessive condensation polymerization for precipitating mesophase, a method of separating and removing improper light or heavy components by means of a solvent, a method of depressing the formation of the heavy components by discontinuing the formation of the mesophase and separating the anisotropic components and the light components on settling and the like. In addition, other processes which are directed to obtaining a preferable structure for spinning by improving the fluidity of the pitches by means of controlling the molecular weights have been developed including a Domant mesophase method which consists of hydrogenating anisotropic pitch to form isotropic pitch and thermally treating the isotropic pitch to convert into the anisotropic pitch and a premesophase method which consists of hydrogenating and thermally treating isotropic pitch.
The research and development of processes of melt spinning, infusibilization and heat treatment employing the precursor pitch thus prepared as well as the development of the raw material are conducted. It is known that the dynamical characteristics of the carbon fibers are remarkably influenced by a method of forming the orientation of the molecules and a cross sectional fiber structure formed during the melt spinning.
Structural parameters of a microscopic structure governing the dynamical characteristics of the pitch-based carbon fibers include the degree of preferential orientation of a carbon layer along a fiber axis, the cross sectional fiber structure, the shape and the size of closed pores, the distance between adjacent carbon hexagonal layers, the thickness of parallel stacked layers, the length of the respective layers, the surface and internal structures, nonuniformity, chemical compositions, existence of impurities and the like.
On the other hand, a macroscopic fiber structure is deeply related to properties of a fiber, and a cross sectional shape of a fiber and macroscopic orientation of carbon layers considerably influence the dynamical characteristics. It is realized that the optically anisotropic carbon fibers are likely to form relatively broad layers, and for example if its orientation of the fiber cross section possesses a radical structure, cracks are liable to be created along the fiber axis during the heat treatment to largely decrease the strength. The factors dominating the said orientation depend on, as mentioned earlier, the raw material, the temperature of the spinning and the structure of a spinning nozzle.
The spinning conditions influence the orientation of the carbon layers; e.g., the orientation is determined by the temperature of the pitch, the change of the flow circumstances of the melted pitch flowing through the spinning nozzle based on the structure of the spinning nozzle, and the carrying out of a thinning step of the fibers discharged from a discharge opening.
The orientation of the molecules constituting the pitch at the time of spinning is generally known to be perpendicular to the wall surface of the spinning nozzle and parallel to a free interface of a gas and the like by means of surface tension. Since the spinning nozzle generally possesses a circular or deformed cross section and the raw material is discharged through the nozzle, the spun fibers are likely to have a radial structure perpendicular to the wall surface of the spinning nozzle. This radial structure likely to be produced especially in the case of the circular cross section, is liable to create cracks in the subsequent infusibilizing and heat treating processes, and is accompanied by many problems for elevating the mechanical strength.
Various methods have been developed which prevent the formation of the cracks due to the radial structure of the carbon fibers obtained from the optically anisotropic pitch. The representative ones include a method in which metallic or inorganic crushed powders, fine powders or ultra-fine sintered powders are packed in the introduction part of a nozzle as shown in Japanese patent laid open gazette No. 61-258023 and a method in which a non-porous longitudinal molded element for forming a space constituting a path for a melt is located in an introduction opening as shown in Japanese patent laid open gazette No. 60-259609. The both methods intend to obtain carbon fibers having the random structure or the like with no cracks by means of controlling the flow of pitch in the introduction opening.
However, in reality, a nozzle with plural openings should be employed for industrially preparing the carbon fibers so that it is quite difficult to make uniform the pressure drops of the respective opening in the former method consequently resulting in a problem that stable spinning cannot be achieved due to the nonuniformity of the fiber diameters of the respective openings. On the other hand, since, in the latter method, the molded element in the introduction opening forms the path for the melt between the molded element and the inner wall of the opening, the cross sectional area of the path for the melt is naturally much smaller than that of the introduction opening to inevitably raise the spinning pressure. Further, the cost of preparing the molded element having a particular shape may be quite high.